An experimental investigation of head loss through a triangular ‘‘V- shaped” screen

Common traditional screens (screens perpendicular and vertical to the flow direction) face extensive problems with screen blockage, which can result in adverse hydraulic, environmental, and economic consequences. Experimentally, this paper presents an advanced trash screen concept to reduce traditional screen problems and improve the hydraulic performance of screens. The traditional screen is redeveloped using a triangular V shape with circular bars in the flow direction. Triangular V-shaped screen models with different angles, blockage ratios, circular bar designs, and flow discharges were tested in a scaled physical model. The analyses provide promising results. The findings showed that the head loss coefficients were effectively reduced by using the triangular V-shaped screens with circular bars (a < 90 ) in comparison with the traditional trash screen (a = 90). Additionally, the results indicated that the head loss across the screen increased with increasing flow discharge and blockage ratio. The losses considerably increase by large percentages when the screen becomes blocked by 40%. Low head losses were recorded at low screen angles for the circular bars. A new head loss equation is recommended for triangular screens with circular bars.

Original Article

An experimental investigation of head loss through a triangular

‘‘V- shaped” screen

Mahmoud Zayeda,⇑, Anas El Mollab, Mohammed Sallaha

a Channel Maintenance Research Institute, National Water Research Center, Egypt

b

Faculty of Engineering, Irrigation and Hydraulic Department, Al-Azhar University, Egypt

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 11 September 2017

Revised 20 December 2017

Accepted 26 December 2017

Available online 27 December 2017

Keywords:

Triangular trash screen

Head losses

Experimental hydraulics

Channel

Open channel flow

a b s t r a c t Common traditional screens (screens perpendicular and vertical to the flow direction) face extensive problems with screen blockage, which can result in adverse hydraulic, environmental, and economic con-sequences Experimentally, this paper presents an advanced trash screen concept to reduce traditional screen problems and improve the hydraulic performance of screens The traditional screen is re-developed using a triangular V shape with circular bars in the flow direction Triangular V-shaped screen models with different angles, blockage ratios, circular bar designs, and flow discharges were tested in a scaled physical model The analyses provide promising results The findings showed that the head loss coefficients were effectively reduced by using the triangular V-shaped screens with circular bars (a< 90°) in comparison with the traditional trash screen (a= 90) Additionally, the results indicated that the head loss across the screen increased with increasing flow discharge and blockage ratio The losses considerably increase by large percentages when the screen becomes blocked by 40% Low head losses were recorded at low screen angles for the circular bars A new head loss equation is recommended for triangular screens with circular bars

Ó 2018 Production and hosting by Elsevier B.V on behalf of Cairo University This is an open access article

under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Introduction Trash screens are commonly used to trap debris in streams Debris can accumulate around structures and cause structural fail-ure[1,2], impede the waterway openings (culverts, bridges, etc.),

https://doi.org/10.1016/j.jare.2017.12.005

2090-1232/Ó 2018 Production and hosting by Elsevier B.V on behalf of Cairo University.

Peer review under responsibility of Cairo University.

⇑ Corresponding author.

E-mail address: mahmoudzayed13@yahoo.com (M Zayed).

Contents lists available atScienceDirect Journal of Advanced Research

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j a r e

adversely affect power generation intakes and flood control

pro-jects [3], increase the navigation problems[4], and increase the

upstream flood risk potential[5] Trash screens are used to prevent

such hazards associated with debris accumulation As reported by

Blanc[6], controlling debris using trash screens has been studied

by numerous investigators[7–11]

The challenges related to trash screens are that they can be

blocked by debris accumulations and can cause head losses Trash

screen blockage and losses can have serious hydraulic,

environ-mental and economic consequences The accumulation process

can quickly occur, especially during flood periods; at the same

time, the prediction of potential screen blockage is incredibly

diffi-cult[12]

In particular, debris enlargements produce improper screen

functioning, resulting in a possible increase in the upstream water

level, obstructions to flow across screens, and extensive flooding

[6,12] Moreover, debris accumulation at the screen can be a

signif-icant problem that increases the downstream velocity and creates

scouring[13], breaks down turbines and hydroelectric generation

plants [14,15] and yields incorrect predictions for irrigation

engineers

Head loss is a vital factor in how a trash screen is designed The

head loss across the screen significantly increases after it becomes

blocked[12] Notably, the screen head loss is the major part of the

total head loss[12] Flow through trash screens has been

investi-gated by various researchers[16–26], and previous studies treated

the screen as a traditional screen without a shape, i.e., only a

per-pendicular–vertical screen inclined from the bed or angled from

the wall

Meusburger[27], as cited by Raynal et al.[25], proposed a head

loss formula for an angled screen, as given in Eq.(1) In this

equa-tion, the screen angle and blockage ratio are coupled considering

the bar shape coefficient presented by Kirschmer[16]and without

assessing the relation between the bar shape and the rack angle

Dh¼ K B

1 B

1 :5 a

90

B1:4 tanð90aÞ v2

2g



ð1Þ

whereDh is the head loss, K is the bar shape coefficient presented

by Kirschmer[16], B is the blockage ratio,ais the screen angle from

the wall, v is the approach flow velocity, and g is gravitational

acceleration

Clark et al [28] investigated tests of straight and oblique

approach flows Eq.(2)was developed for the angle of the trash

screen The tests examined the effect of the bar shape, angle of

the approach flow and blockage ratio

Dh¼ 7:43gð1 þ 2:44 tan2hÞB2 v2

2g



ð2Þ

where Dh is the head loss, g is the bar shape factor, h is the

approach flow angle, B is the blockage ratio, v is the approach flow

velocity, and g is gravitational acceleration

Wahl [23] presented Eq (3) for calculating the head loss

through screens regardless of the screen angle and bar shape

Dh¼ ð1:45  0:45D  D2Þ v2

2g



ð3Þ

whereDh is the head loss, D = 1 – B, B is the blockage ratio, v is the

approach flow velocity, and g is gravitational acceleration

Available formulae for calculating screen head loss under

differ-ent settings have been presented by various researchers

[19,25,26,29–31]

A number of other studies have also been performed, some

examples of which include an experimental investigation of flow

through vertical angled screens in a diversion structure[32], water

energy dissipation using vertically placed screens [33] and an investigation of energy loss due to open channel contractions[34] This paper presents a screen development concept that maxi-mizes the hydraulic performance and reduces the hydraulic prob-lems caused by traditional screens (perpendicular–vertical screen, defined asa= 90° and b = 90° based on the direction of flow) In this method, the screen was designed with a triangular V shape based on the flow direction Additionally, success criteria that gov-ern the screen conditions and a new head loss equation are introduced

A series of experiments were performed on a hydraulic physical scale model at Channel Maintenance Research Institute Hydraulics Laboratory at the National Water Research Center (NWRC), Egypt Based on different screen wall angle configurations shapes, and blockage ratios, the various results were analyzed

Methodology Experimental setup All experimental runs were conducted with a trash screen model in a recirculating, 16.22 m long, 0.6 m wide, 0.42 m deep and 1:1 side slope horizontal trapezoidal open channel made of concrete The flume was attached with a head tank A constant underground reservoir was provided to supply the flume with water through a 5-inch pipe Then, the water entering the flume was drained to the underground reservoir The flow was circulated through the system by two 5-in centrifugal pumps driven by a motor A tailgate was attached at the downstream end of the chan-nel to adjust the water levels The flow discharges, which were adjusted via a discharge valve, were measured with a current flow meter, and a mobile point gage was used to measure the water depths to the nearest ±1 mm

The model was scaled to simulate the most appropriate method

In the model, four angles were tested: 90° to replicate the tradi-tional popular screen and angles of 75°, 65°, and 55° to verify the influence of the screen angle compared to that of the traditional screen Flow discharges ranging between 20 L/s and 40 L/s were applied Ideally, the selected ranges of the discharges represented relatively low, moderate, and high flows that could be created with the experimental test rig under steady flow conditions The screens were tested under wide ranges of blockage ratios between 0.10 and 0.66 to cover different conditions of blockage simulation The screen models, which were vertically installed to the chan-nel bed (b = 90°), were angled from the wall and shaped to form tri-angular or V-shaped screens (seeFig 1) Four screen angles were tested; the smallest angle wasa= 55°, and the other angles were

a= 65°, 75°, and 90° (perpendicular to the channel) to the flow direction All screens were 25 cm in height, whereas the lengths were different as a result of the screen angles The screens were composed of circular mild steel bars 3 mm in diameter and with

2 cm spacing The circular bars were used to make the screen angle the same in any direction

Four different blockage ratios (0.10, 0.29, 0.47, and 0.66) were used for each screen model The blockage consists of two main ele-ments: blockage of all parts of the immersed bars and blockage of the row of wooden sheets located at the top of the wetted screen The blockage ratio (B) can be described by Eq.(4) All blockage ele-ments were coupled between the lateral screen bars and the row of wooden sheets

B¼Ab

where B is the blockage ratio, Abis the total area of the immersed blockage, and A is the area of the channel

Five discharges, Q = 20 L/s, 25 L/s, 30 L/s, 35 L/s, and 40 L/s, were

used for each screen arrangement A 3-D Vectrino device was used

to measure the approach flow velocity To minimize the potential

errors of fluctuations in the flow rate, an appropriate length of time

surpassed to establish steady conditions The conditions were

allowed to stabilize for 15 min before testing

Experimental procedure

The considered parameters, screen angles of the triangular

screens from the wall, unit discharge, and blockage ratio were

carefully assessed Throughout the experiments, each screen angle

was examined for each discharge and blockage ratio After fixing

the screen model at the middle of the flume downstream of the head tank, the tailgate was adjusted to predefine the gate opening Then, flow passed through the system In both the center of the flume and near the wall of the flume, the upstream and down-stream water depths (h1 and h2, respectively) were measured every 15 cm near the screen and then every 50 cm Then, the aver-age of h1and h2was calculated to avoid errors for each configura-tion The test was repeated again with another blockage ratio Then, the experiment was repeated with another screen angle Finally, the head loss was calculated, and the head loss coefficient was extrapolated using Eq.(5)

Dh¼Dhc v2

2g



; Dhc¼ v2D=2gh

ð5Þ

(a)

(b)

(c)

2 1

Upstream water level

Downstream water level

Bed level

Trash screen

β

Q

Triangular trash screen

1:1

1:1

Side slope Side slope

Wall

Fig 1 Diagram of the triangular screen in the channel: (a) plan details for a triangular screen from the wall (not to scale), (b) picture of a triangular screen, (c) side view of the details of the triangular screen from the channel bed (b = 90°) (neither to scale nor to geometric projection).

whereDh is the head loss,Dhcis the head loss coefficient, v is the

approach flow velocity, and g is gravitational acceleration

To minimize any errors associated with the experimental

condi-tions, the measurements were evaluated at different points during

each session Furthermore, the test sets did not always begin at the

same time of the day or week to reduce any possible

environmen-tal effects, such as changes in fluctuations and water temperature

In terms of accuracy, the tested measurements were repeated to

estimate the uncertainty The mean absolute error associated with

measurements did not exceed 1.23%

Non-dimensional analysis

The effects of the triangular screen angles, unit discharge, and

blockage ratio on the head losses were studied for various settings

To generalize the experimental observations, the head loss and the

main evolution factors were reported with non-dimensional

vari-ables The factors that affected the head loss can be defined as

shown in Eq.(6):

whereDh is the head loss, q = Q/b is the unit discharge, Q is the flow

discharge, b is the channel width, v is the approach flow velocity,a

is the screen angle from the wall, B is the blockage ratio, and g is

gravitational acceleration

The head loss coefficient (Dhc) was expressed above in Eq.(5)

By applying non-dimensional terms using the Buckinghamp

theo-rem[35], Eq.(7)can be derived

Dh

v2=2g

whereDh/(v2/2g) =Dhcis the head loss coefficient,Dh is the head

loss, qg/v3is a dimensionless discharge that is expressed as a

dis-charge coefficient, q is the unit disdis-charge, g is gravitational

acceler-ation, v is the approach flow velocity,ais the screen angle from the

wall, and B is the blockage ratio

To identify the state of the flow during the experiments, the

Froude number (Eq (8)) was applied; based on the method of

Chow[36] The results indicate that Frranges between 0.044 and

0.12, suggesting that the state of the flow was subcritical

Fr¼ vffiffiffiffiffiffiffiffi

gh1

where v, g, and h1 are the approach flow velocity, gravitational

acceleration, and upstream water depth, respectively

Results and discussion

A sensitivity analysis was conducted to assess how the

triangu-lar screen angles, unit discharge, and blockage ratio affect the

screen head losses

Effect of the triangular screen angle

Experiments were performed for screen angles (a= 55°, 65°, 75°

and 90°) to assess the associated impact on the screen head losses

The results of the screen angle variations are reported inFig 2for

different test arrangements The effect is particularly evident in

Fig 2, which shows that the triangular screen angles strongly affect

the head loss coefficients For each blockage ratio,Dhcdecreases

with decreasing triangular screen angle (a) In other words, the

large gaps in the triangular screen with circular bars have a higher

tendency to reduce the head loss coefficients than do small gaps

These results were potentially because that at high screen lengths, the distribution area of the flow increases and the head loss decreases Clearly, the results are similar for all the discharge coefficients

Likewise, the experimental data from analyses of traditional horizontal screen (a= 90°) head losses were compared with those collected for various triangular screen angles The analysis showed the following results for screen angles of 75°, 65° and 55°: (1) for a blockage ratio of 0.10, the head lossDh decreased by 75%, 79.5%, and 85%, respectively; (2) for a blockage ratio of 0.29,Dh decreased

by 41%, 51%, and 70.6%, respectively; (3) for a blockage ratio of 0.47,Dh decreased by 34.5%, 47.7%, and 65.5%, respectively; and (4) for a blockage ratio of 0.66,Dh decreased by 20%, 32.6%, and 49%, respectively Therefore, it is evident that by increasing the blockage ratio, the effect of the screen angle decreases

In summary, the head losses are decreased by using a triangular screen (a< 90°) with circular bars compared with the traditional horizontal screen (a= 90°)

Based on the experimental results of the head losses, a paired t-test statistical analysis was used to define whether there was a statistically significant difference in the head losses found for the tested screen angles under various conditions The paired t-test results are presented inTable 1 As shown inTable 1, significant differences between the screen angles according to the head loss values were found for all scenarios Consequently, the results con-firm that the screen angle is a valuable element in practical appli-cations, and it clearly influences the head loss

Effect of discharge For different screen angles and blockage ratios, a wide range of discharge values (20 L/s, 25 L/s, 30 L/s, 35 L/s, 40 L/s) was estab-lished during the experiments to consider the associated effect

on head losses The non-dimensional discharge is defined as the discharge coefficient equal to qg/v3.Fig 3shows the relationships between the dimensionless Buckinghampterm head loss coeffi-cient Dhc and the non-dimensional discharge coefficient qg/v3 (303, 211, 142, 91, and 63) for triangular screen angles of 55°,

65°, 75° and 90° and blockage ratios of 0.10, 0.29, 0.47 and 0.66 The results of these tests indicate that as the non-dimensional dis-charge coefficient increases, the head loss coefficient Dhc also increases Furthermore, lowDhcvalues result in consistently low screen angles for different blockage ratios As a result, it is evident that head loss increases with discharge (and thus the approach flow velocity) Therefore, the flow discharge is considered an important parameter, and the head loss is a function of discharge Effect of the blockage ratio

The blockage ratios (0.10, 0.29, 0.47, and 0.66) were analyzed carefully for all the tested screen model arrangements.Fig 4 pre-sentsDhcas a function of B for tests with a triangular screen angle equal to 55°, 65°, 75° and 90° and different discharge coefficients For all the screen angles and discharge coefficients evaluated, the results show thatDhcrapidly increases with B, as expected; how-ever, this increase becomes less notable as the screen angle decreases In addition,Dhcis similar for all the tested screen mod-els, and for screen blockage ratios greater than 40%,Dhc consider-ably increases A paired t-test statistical analysis was used to determine the statistically significant difference between screen blockage ratios based on the obtained head loss values, and the results are detailed inTable 2.Table 2shows that significant statis-tical differences exist between the blockage ratios according to the

head loss values for all screens In fact, for all the tested models, the

head loss is a function of the blockage ratio

Derivation of a new empirical equation

The noted influential factors have been identified, considering

the non-dimensionality of terms, to develop a new equation for

estimating the head loss for triangular screens with circular bars

Multivariable regression analysis was applied, and the parameters

were correlated to establish the new head loss Eq (9)at a 95%

probability significance level

Dh

v2=2g

¼ 11:45 qgv3

 0:25

From a statistical perspective, the model had a high adjusted

determination coefficient (R2) value of 0.95, indicating that it

exhibited a good fit with the experimental test data All contribut-ing factors were found to be significant predictive factors, whereas all non-dimensional factors had P-values < 0.0001 The form of

Eq.(9) indicates that the screen head loss involves three terms: the non-dimensional discharge, the screen angle, and the blockage ratio Therefore, the screen head loss can easily be obtained by applying the proposed equation in the tested range of parameters

Fig 5shows a comparison of the measured head loss coefficients and those predicted by Eq (9) The comparison yielded a high determination coefficient (R2) of 0.96 Thus, the derived Eq.(9)is

an effective equation of head loss prediction The developed equa-tion is applicable to triangular screens or V-shaped screens inserted in a straight channel based on the flow direction, with

an angle from wallabetween 90° and 55°, blockage ratio between 0.10 and 0.66, and circular bars It could be notable to verify the current study by numerical verification in future works

Fig 2 Relationships between the head loss coefficients and screen angles for different discharge coefficients and blockage ratios of (a) 0.10, (b) 0.29, (c) 0.47, and (d) 0.66.

Table 1

Paired t-test of different screen angles based on head loss standards (critical (P = 0.05)).

Blockage ratio Screen angle (Degrees) Screen angle (Degrees) t-state P-value Significantly different

Fig 3 Relationships between the head loss coefficients and discharge coefficients (qg/v 3 ) for different screen angles and blockage ratios of (a) 0.10, (b) 0.29, (c) 0.47 and (d) 0.66.

Fig 4 Relationships between the head loss coefficients and blockage ratios for different discharge coefficients and screen angles of (a) 90°, (b) 75°, (c) 65° and (d) 55°.

This research analyzed the experimental and statistical results

of using triangular screens to investigate how the hydraulic

problems produced by screen blockage can be reduced Different

screen angles with circular bars, blockage ratios, and discharges

have been identified The results and conclusions of the analysis

are as follows:

 This paper produced a detailed methodology that can be useful

in assessing the performances of different trash screen

arrangements

 The contributing parameters, screen angles, blockage ratios, and

discharges were analyzed based on their influence of the screen

head loss

 The results indicated that a low screen angle leads to a low screen

head loss coefficient, whereas high blockage ratios will decrease

the effect of the screen angle In other words, increasing the

trian-gular screen lengths by decreasing the screen gaps will reduce

the screen head loss coefficient; thus, the triangular screen angle

(a< 90°) can decrease the head loss coefficient in comparison

with the common traditional horizontal screen (a= 90°)

 A low head loss coefficient will yield a low non-dimensional

discharge; at the same time, a low screen angle will lead to a

low head loss coefficient

 The head loss is a function of the blockage ratio For all the

non-dimensional discharges, the screen head loss coefficient rapidly

increased with the blockage ratio; however, at low screen

angles, lowDhcvalues were generally obtained

 When the screen was blocked by 40% or more,Dhcwas

gener-ally high

 Statistically, the results indicate that significant differences between the screen angles and blockage ratios are found for all screen considerations based on the head loss values

 Because multiple contributing parameters (screen angle, block-age ratio, and discharge) directly influence the practical head loss of a screen, head loss can be considered a vital factor in assessing the hydraulic performance of a screen

 A new equation (Eq.(9)) was derived for the proposed method This equation can be used to estimate the head loss of triangular screens with circular bars

 Triangular V-shaped screens may be more likely to deflect float-ing matter to the channel sides without human interference (self-cleaning screens) This result may be due to the water excitation forces that affect the debris orientation toward the sides and facilitate the flow of the waterway through the screen

 The results provide a better understanding of triangular screen blockage and can help in designing triangular V-shaped screens with circular bars

Notation

Ab area of immersed blockage

Ac area of the channel

B blockage ratio = Ab/Ac

b channel width

Fr Froude number of upstream flow

g gravitational acceleration

h1 upstream water depth

h2 downstream water depth

K bar shape coefficient presented by Kirschmer[16]

Q flow discharge

q unit discharge

v approach flow velocity

g bar shape factor

a trash screen angle from the wall

b trash screen angle from the channel bed

h approach flow angle

Dh head loss through the trash screen

Dhc trash screen head loss coefficient Conflict of interest

The authors have declared no conflict of interest

Compliance with Ethics Requirements This article does not contain any studies with human or animal subjects

Table 2

Paired t-test of different blockage ratios based on head loss standards (critical (P = 0.05)).

Screen angle (Degrees) Blockage ratio Blockage ratio t-state P-value Significantly different

Fig 5 Comparison of measured Dh c values and those predicted by Eq (9)

This work was conducted at the Hydraulic Laboratory of

Chan-nel Maintenance Research Institute (CMRI), the National Water

Research Center (NWRC), Egypt The authors greatly appreciate

the support of the CMRI

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